The VCA (Voltage Controlled Amplifier) is a type of construction with a somewhat ambiguous name, because all the dynamics processors are controlled in voltage, but in this case the acronym indicates an integrated circuit incorporated in a single container.
The first VCA appeared in 1971 and was created by DBX Inc. was the 202 model. The first models consisted of a circuit a discrete transistors, selected in high temperature, and mounted together with other passive components on relatively small boards. The board was then mounted in a metal container, designed to shield it from external electromagnetic interference and sealed with silicone to uniform the temperature coefficient of the components. The only disadvantage of the first VCAs in history was a certain thermal instability and high distortion generation at the level of odd harmonics. But this technology developed very quickly and in a few years it came to have integrated cards more and more miniaturized, up to the 1996 when Dave Derr di Empirical Labs placed on the market the Distressor which is the first digitally controlled analog VCA compressor. The use of the VCA brings considerable advantages in the design of a compressor: first of all the total predictability in the behavior of the components, which does not rely on the peculiarity of the circuit as in the previously described technologies, the remarkable linearity in the frequency response, greater reliability , reduced harmonic distortion all enclosed in small dimensions. In general, their principle of operation uses the input signal (feed forward) or the output signal (feedback) as the source for the control voltage.
Among the most used VCA compressors over the years and which have become "famous" we certainly find the bus compressor di SSL e 2500 API.
These two machines are similar in the scheme of operation and in the fact that all their connections are balanced and have the bypass of the machine, but they differ in some characteristics.
Solid State Logic
SSL is a stereo compressor that uses a proprietary op-amp and its controls are: attack, ratio and release with jerky potentiometers, threshold that goes up to +15dBu and make up gain, with continuous potentiometers. Unlike the other processor, the bus compressor has the auto mode on the release time which sets it to a value close to two seconds. The meter displays only the gain reduction, furthermore we find a switch to activate the stereo input for the sidechain from an external source and another switch that allows the auto fade of the compressor on the audio signal, controlled by a click potentiometer. on factory preset values ranging from 1 - 60 seconds.
2500 API
the API 2500 is a stereo compressor, which also uses a proprietary op-amp, with linked controls for each channel that can operate in dual mono with the same settings. It has two meters that display the gain reduction of the individual channels, the input and output signal, the other parameters are the same as for SSL, but here they all consist of click potentiometers. The side chain input is stereo.
The two important innovations are the automatic setting of the output level, which makes sure that the user takes care of processing the signal, while the machine will always keep the output signal level constant and, the TONE section which includes the parameters hard, soft and med knee, which regulate the hardness of the compressor intervention, the thrust detector circuit that allows the machine to work on certain ranges of the audio band which unlike the other models here is adjustable by the user and finally, the OLD or NEW style which differs in use, in the first case of a feedback signal, while in the second mode a feed forward signal will be used as the source for the control voltage.
Paolo Alessandro Andrea Cherubini Barberini
